Apparatus for manufacturing carbon nanotube pellets

ABSTRACT

The apparatus for manufacturing carbon nanotube pellets according to the present invention provides carbon nanotube pellets with increased apparent density by using only a small amount of solvent. The carbon nanotube pellets produced by the apparatus according to the present invention can improve various problems generated by scattering of powders. And since the density of the pellet form is high, transport, transfer and improvement become easier. Therefore, it can be more effectively applied to the manufacturing of composite materials.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This application claims the benefit of priority to Korean PatentApplication No. 10-2016-0006939, filed on Jan. 20, 2016, the entiredisclosure of which is incorporated herein by reference.

The present invention relates to an apparatus for manufacturing carbonnanotube pellets, and more particularly, an apparatus for manufacturingcarbon nanotube pellets which can improve the quality of the pellets andthe dispersibility of the carbon nanotubes contained in the pellets.

2. Description of the Related Art

Carbon nanotubes exhibit insulating, conducting or semiconductingproperties depending on their inherent chirality. Carbon nanotubes havea structure in which carbon atoms are strongly covalently bonded to eachother. Due to this structure, carbon nanotubes have a tensile strengthapproximately 100 times greater than that of steel, are highly flexibleand elastic, and are chemically stable. Carbon nanotubes are ofindustrial importance in the manufacture of composites because of theirsize and specific physical properties. Carbon nanotubes can findwidespread applications in numerous fields, including electronicmaterials and energy materials. For example, carbon nanotubes areapplicable to secondary batteries, fuel cells, electrodes ofelectrochemical storage devices (e.g., supercapacitors), electromagneticwave shields, field emission displays, and gas sensors.

However, due to the low density of the bulk carbon nanotubes andscattering in the process due to the powder form of tens of micrometersof the carbon nanotubes, it may cause harm to the human body andmalfunction of the electric appliance. In addition, there is adifficulty in dispersion due to a difference in apparent bulk densitybetween pellets and powder-type polymers to be mixed.

For the above reasons, conventionally, the carbon nanotubes are usuallyprovided by pelletization because of the increase in the density of thecarbon nanotubes and the ease of handling and transportation thereof. Inaddition, the pelletized carbon nanotubes are convenient for use invarious processing apparatuses. In the conventional method, in order togranulate or pelletize the carbon nanotubes, two different methods, thatis, a method in which they are wet pelletized and then dried and amethod in which they are dry pelletized are used.

Generally, dry pelletization uses a pelletizing drum comprising ahorizontally disposed rotary tube, the interior of which is referred toas a pelletizing chamber. In order to granulate the carbon nanotubepowder, it is produced by process in which the industrial powders arepreliminarily densified and rolling from a rotating tube wall in apelletizing drum to pelletize them. They are agglomerated byelectrostatic forces and Van-Der-Waals forces that enable drypelletization and are usually produced by applying a few tons ofpressure during dry pellet formation. Therefore, there is a problem thatthe pellets can be destroyed again during the manufacturing process. Thewet pelletization process is mainly performed by a liquid bridge and acapillary force between the carbon nanotubes. Conventionally, whenmixing with carbon nanotubes by a wet pelletization method, excessivewater is added because the distribution of water and binder is poor. Inthis case, the added water is usually removed by heat in a rotary drumdryer. Excessive water therefore increases the load on the dryer and,consequently, reduces the throughput of the product through the process.Excessive water also increases the energy and time required for drying.Therefore, uniform distribution of water and binder in the carbonnanotube mixture is very important in the pelletization process.Furthermore, if the components of the pellets are not mixed uniformly,the quality of the produced carbon nanotube pellets may not be constant.

To solve this problem, a method of treating a dispersant such as asurfactant in order to improve the dispersibility of pellets has beenstudied. However, there is still a problem that such a material may actas an impurity by remaining in the carbon nanotube pellets.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus formanufacturing carbon nanotube pellets in which carbon nanotube pelletscan be produced with only a small amount of solvent.

Another object of the present invention is to provide a method forproducing carbon nanotube pellets by using the apparatus.

In order to solve the problems of the present invention, there isprovided an apparatus for manufacturing carbon nanotube pelletscomprising:

a mixing part having a mixing device for mixing carbon nanotubes and asolvent;

a kneading part provided at the bottom of the mixing part andadditionally kneading a mixture of the carbon nanotubes and the solventto prepare a carbon nanotubes paste; and

an extruding part for receiving the mixture from the kneading part andforming the mixture into pellets by compression molding.

In order to solve another problems of the present invention, there isprovided a method for producing carbon nanotube pellets by using theabove apparatus, comprising the steps of:

mixing carbon nanotubes and a solvent at a weight ratio of 5:1 to 1:2 inthe mixing part;

kneading the mixture additionally in the kneading part to prepare acarbon nanotube paste; and

extruding the carbon nanotube paste into pellets in the extruding part.

The apparatus for manufacturing carbon nanotube pellets according topresent invention can produce carbon nanotube pellets only with a smallamount of solvent. According to the present invention, the apparentdensity of the carbon nanotubes can be increased, the particle size ofthe carbon nanotubes contained in the pellets can be reduced and thedispersibility to the solvent can be improved. Also, the carbonnanotubes provided in the form of pellets according to the presentinvention can improve the problems of scattering of powders in theprocess for manufacturing composite material and have a high compressionratio over the apparent density of the initial carbon nanotubes.Therefore, it can be easily transferred and handled, and thedispersibility to the solvent is excellent so that the productivity canbe improved in the production of the carbon nanotube composite material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows various embodiments of the apparatus for producing carbonnanotube pellets according to the present invention ((a) an integratedtype, (b) an independent type).

FIG. 2 shows a particle size distribution of carbon nanotube powders anda particle size distribution of pelletized carbon nanotubes in Examplesand Comparative Examples.

FIG. 3 shows dispersion characteristics to the solvent of the pelletsprepared in Examples and Comparative Examples.

FIG. 4 is a graph showing (a) a particle size distribution and (b) aparticle size change over time in the premixing process using CNTs.

FIG. 5 is a graph showing (a) a particle size distribution and (b) aparticle size change over time in the premixing process using CNTpellets.

FIG. 6 shows a change in particle size in a premixing process using CNTsand CNT pellets.

FIG. 7 shows (a) a particle size distribution and (b) a shape ofdistribution of carbon nanotubes after pastes of premixed CNTs and CNTpellets are discharged from a screw mixer.

FIG. 8 shows (a) a particle size distribution and (b) a shape ofdistribution according to the discharging rate increase and the numberof repetitions of screw mixing process.

FIG. 9 shows shapes of distributions of (a) CNTs and (b) pressed CNTpellets after premixing process.

FIG. 10 shows an initial particle size distribution of CNTs without thepremixing process according to the number of times of passing CNTpellets through an extruder.

FIG. 11 shows an initial particle size distribution of CNTs after thepremixing process according to the number of times of passing CNTpellets through an extruder.

DETAILED DESCRIPTION OF THE INVENTION

It should be understood that the terms and words used in thespecification and claims are not to be construed as having common anddictionary meanings, but are construed as having meanings and conceptscorresponding to the spirit of the invention in view of the principlethat the inventor can define properly the concept of the terms and wordsin order to describe his/her invention with the best method.

The present invention will now be described in detail.

The present invention relates to an apparatus for manufacturing carbonnanotube pellets having improved dispersibility and density. Theapparatus according to the present invention comprises:

a mixing part having a mixing device for mixing carbon nanotubes and asolvent to prepare a carbon nanotubes paste;

a kneading part provided at the bottom of the mixing part andadditionally kneading a mixture of the carbon nanotubes and the solvent;and

an extruding part for receiving the mixture from the kneading part andforming the mixture into pellets by compression molding.

The mixing device provided in the mixing part may be a mixing deviceused in a general apparatus for producing slurry, and may be selectedfrom a mixing apparatus such as an agitation type, a shaking type, or arotary type. Specifically, there may be mentioned a dispersion kneadersuch as a homogenizer, a ball mill, a sand mill, a roll mill, aplanetary mixer and a planetary kneader. It may be preferred that thestirring speed in a vertical direction and a horizontal direction can beindependently controlled.

According to one embodiment, the screw mixer may include a single screw,a double screw, and multiaxial screw such as a three-axial screw ormore.

According to one embodiment, the apparatus has an integrated type (FIG.1a ) in which the kneading part may be located in the mixing part, or anindependent type (FIG. 1b ) in which the kneading part is separated fromthe mixing part and the mixed solution produced in the mixing part issupplied to the kneading part. It can be selected depending on themanufacturing method.

For example, in the manufacturing apparatus, the mixing part and thekneading part are separated from each other and the mixing step by themixing part and the kneading step by the kneading part may beindependently performed. Specifically, the apparatus may have aseparator between the mixing part and the kneading part. Alternativelythe apparatus itself may be divided so that the carbon nanotube mixtureis produced in the mixing part, and then the produced mixture may besupplied to the kneading part. Alternatively, a hopper may be furtherprovided between the mixing part and the kneading part and a separatormay be provided between the hopper and the mixing part, so that there isan independent space in which the mixing part and the hopper areisolated by the separator. In the above-mentioned independent type ofmanufacturing apparatus, it is possible to carry out a continuousprocess in which a step in which the carbon nanotube mixture is suppliedto the hopper or the screw device, then the mixture supplied from themixing part is fed into the screw mixer, is kneaded and is transferredto the extruder to produce pellets, and a step in which the carbonnanotube and the solvent are supplied to the mixing part to prepare amixture, are simultaneously proceeded.

The extruder can be used without limitation as long as it allows to moldslurry or paste. For example, a screw extruder may be used, and theextruder may include a molding part for molding into the pellets havinga predetermined length and diameter.

According to the present invention, by using the above apparatus, carbonnanotube pellets by using only a small amount of solvent can beproduced. The carbon nanotube pellets according to the present inventionare produced by mixing carbon nanotubes and a solvent at a weight ratioof 5:1 to 1:2 and extruding it, and may have an apparent density of 90kg/m³ or more.

The solvent to be added to the mixing step may be initially added or maybe added in portions according to the process steps. Alternatively, thecarbon nanotubes may be dispersed in a large amount of the solventcompared to said content of the solvent in the mixing process and thenonly the solvent is extracted so that they may be mixed at a highconcentration. In this case, the content of the solvent may be theamount of the finally remaining solvent.

The apparent density of the carbon nanotube pellets according to thepresent invention is remarkably increased compared to the apparentdensity of the carbon nanotubes in the powder form. The apparent densitymay be 90 kg/m³ or more, preferably 100 kg/m³ or more, and morepreferably 120 kg/m³ or more. In addition, the apparent density may be250 kg/m³ or less. Alternatively it may be 230 kg/m³ or less, or 200kg/m³ or less.

The present invention can provide carbon nanotube pellets containing thecarbon nanotubes which are extruded from the carbon nanotube pastehaving a high concentration and compressed. According to one embodiment,the compression ratio of the pellets may be defined by the followingExpression 1.

CNT compression ratio (%)=[apparent density of CNT pellet aftercompression]/[apparent density of CNT before compression]  [Expression1]

According to one embodiment, the diameter of the carbon nanotube pelletsmay be 1 mm or more, or 3 mm or more, preferably 4 mm or more, and morepreferably 5 mm or more and may be 20 mm or less, preferably 15 mm orless, and more preferably 10 mm or less. The length of the carbonnanotube pellets may be 10 mm or more, preferably 20 mm or more, morepreferably 30 mm or more, or 50 mm or more. Further, the pellets mayhave a length of 200 mm or less, or 180 mm or less, or 150 mm or less.

The pellets can be manufactured in a wide of variety of shapes,including, but not limited to, chips, pellets, tablets pills, beads,necklaces, etc.

In addition, the carbon nanotubes contained in the carbon nanotubepellets of the present invention may have a reduced particle size viathe extrusion process and the mixing process, for example may have areduced particle size to 60% or less of the average particle size (D50)of the powdery carbon nanotube, and preferably to 50% or less.

Therefore, the average particle size (D50) of the carbon nanotubeparticles contained in the carbon nanotube pellets may be about 200 μmor less, or about 150 μm or less. And it is possible to contain carbonnanotubes having the average particle size of about 20 μm or less, andpreferably about 15 μm or less depending on the method of themanufacturing process.

The present invention also provides a method for producing carbonnanotube pellet as described above.

According to the method for producing the carbon nanotube pelletsaccording to the present invention, the method comprises the steps of:

mixing carbon nanotubes and a solvent at a weight ratio of about 5:1 toabout 1:2 in a mixing part;

further kneading the mixture to prepare a carbon nanotube paste in akneading part;

extruding the carbon nanotube paste into pellets in an extruding part;and

drying the pellets.

The carbon nanotube paste may be further mixed in a kneader equippedwith a screw mixer after a mixing step with a general type of stirrerand then transferred to an extruder.

For example, the carbon nanotube pellet may be produced by the methodcomprising the steps of:

premixing carbon nanotubes and a solvent;

stirring and mixing the premixed solution of carbon nanotubes in a screwmixer; and

transferring the mixed solution of carbon nanotubes from the screw mixerto an extruder and extruding it into pellets.

According to one embodiment, the mixing in the screw mixer may proceedat a higher concentration than in the premixing step, and thus theviscosity of the mixed solution of carbon nanotubes may be increased ascompared with the primary mixed solution.

According to one embodiment, the premixing may be performed by adding asolvent at a time, or by adding a solvent in several steps. For example,it comprises the steps of:

mixing a small amount of solvent with carbon nanotubes to prepare afirst paste (Kneading);

adding additional predetermined solvent to the first paste and kneadingit to prepare a second paste (Paste); and

adding a solvent to the second paste to prepare a premixed solution(Dilution).

The content of carbon nanotubes in the preliminary mixed solution may be2 to 15% by weight, preferably 2 to 10% by weight, and more preferably 3to 8% by weight based on the total weight of the whole mixed solution.The method may further comprise the step of increasing the concentrationof the mixed solution by partially removing the solvent involved beforemixing in the screw mixer. Alternatively, the concentration may beincreased by removing the solvent as the temperature is raised by ascrew mixing step. The mixed solution of carbon nanotubes which isfinally mixed in the screw mixer may contain the carbon nanotubes andthe solvent in a weight ratio of about 5:1 to about 1:2.

According to one embodiment, in the preliminary mixing process, thestirring time in the first step (kneading) and the second step (paste)may be 2 hours or less, preferably 1 hour or less, respectively. In caseof mixing for more than the above stirring time, the aggregation betweenthe particles may become large, and the dispersion characteristics maybe rather deteriorated.

Finally, the stirring time for the low-concentration carbon nanotubesmay be 30 minutes or more, preferably 60 minutes or more, and morepreferably 100 minutes or more, and the total time of each step of thepremixing process may be 300 minutes or less and preferably 240 minutesor less.

The viscosity of the premixed solution of carbon nanotubes prepared inthe premixing step may be 4,000 cps to 10,000 cps, preferably 5,000 cpsto 10,000 cps.

The premixed solution may be added to a screw mixer to be further mixed.In this process, the concentration of the carbon nanotubes may beincreased, or the premixed solution may be supplied to the screw mixerafter removing some of the solvent. It may be supplied to the extruderafter further mixing in the screw mixer for at least about 5 minutes,preferably at least 10 minutes, more preferably at least 20 minutes. Atthis time, the discharging amount of the mixed solution from the screwmixer can be adjusted to 0.3 kg/min or more, preferably 0.5 kg/min ormore, and can be adjusted to about 5 kg/min or less, preferably 3 kg/minor less, and more preferably 2.5 kg/min or less. Depending on thedischarging rate, the particle size and the degree of dispersion of thecarbon nanotubes contained in the mixed solution may vary, and theviscosity of the mixed solution may also increase.

The viscosity of the mixed solution of carbon nanotubes discharged fromthe screw mixer may be about 9,000 to 30,000 cps.

According to one embodiment, the carbon nanotube pellets producedthrough the extrusion step can be re-dispersed and mixed in a solventand re-extruded to produce pellets. In the process of re-dispersing thecompressed pellets as described above, the particle size and/orviscosity of the carbon nanotubes are reduced, thus the dispersioncharacteristics in the solvent can be improved and the efficiency of themanufacturing process can be improved.

For example, when the premixed solution is prepared by using the carbonnanotubes, the viscosity of the premixed solution can be reduced to4,000 cps or less, and the viscosity of the carbon nanotubes dischargedafter mixing in the screw mixer is lowered to 10,000 cps or less.

In addition, in the manufacturing process for producing carbon nanotubepellets by re-dispersing the carbon nanotube pellets, the size of thecarbon nanotubes may vary depending on the number of times of extrusion.For example, the particle size of D90 of the particles of carbonnanotubes may satisfy the following Expression 2:

−167.3x+650≤y≤−167.3x+670  [Expression 2]

wherein x is the number of times of extrusion of the carbon nanotubes,and y is the particle size of D90 (μm) of the carbon nanotubes.

The mixed solution of carbon nanotubes will now be described in detail.

According to one embodiment, the solvent in which the carbon nanotubesare mixed is at least one selected from the group consisting of water,alcohols, Cellosolve, ketones, amides, esters, ethers, aromatichydrocarbons such as benzene or toluene, and aliphatic hydrocarbons suchas hexane or heptane, and preferably at least one selected from thegroup consisting of water, alcohols, amides, esters, and ketones.

For example, any one selected from the group consisting of water,methanol, ethanol, propanol, acetone, dimethylformamide (DMF),dimethylacetamide, dimethyl sulfoxide (DMSO), and N-methylpyrrolidone(NMP) or a mixed solvent of two or more of them may be used.

The carbon nanotubes according to the present invention may be preparedby chemical vapor deposition (CVD) through decomposition of a carbonsource by using a supported catalyst. The use of the supported catalystallows for growth of the starting carbon nanotubes. The catalytic metalof the supported catalyst is not especially limited so long as itpromotes the growth of carbon nanotubes.

Examples of such catalytic metals include metals of Groups 3 to 12 inthe 18-group type Periodic Table of the elements recommended by IUPAC in1990. The catalytic metal is preferably selected from the groupconsisting of the metals of Groups 3, 5, 6, 8, 9, and 10. Particularlypreferred is at least one metal selected from iron (Fe), nickel (Ni),cobalt (Co), chromium (Cr), molybdenum (Mo), tungsten (W), vanadium (V),titanium (Ti), ruthenium (Ru), rhodium (Rh), palladium (Pd), platinum(Pt), and rare earth elements. A precursor of the catalytic metal mayalso be used. The catalytic metal precursor is not particularly limitedso long as it contains the catalytic metal. For example, the catalyticmetal precursor may be an inorganic salt (e.g., a nitrate, sulfate orcarbonate) of the catalytic metal, an organic salt (e.g., an acetate) ofthe catalytic metal, an organic complex (e.g., an acetylacetone complex)of the catalytic metal, and an organometallic compound of the catalyticmetal.

It is widely known that a reaction activity is controlled by using acombination of two or more catalytic metals and catalytic metalprecursor compounds, for example, a combination of at least one elementselected from iron (Fe), cobalt (Co), and nickel (Ni), at least oneelement selected from titanium (Ti), vanadium (V), and chromium (Cr),and at least one element selected from molybdenum (Mo) and tungsten (W).Preferably, the metal catalyst includes cobalt (Co) as a major componentand optionally one or more metals selected from iron (Fe), molybdenum(Mo), chromium (Cr), and vanadium (V).

Specifically, the catalyst used in the preparation of the startingcarbon nanotubes may be prepared by dissolving Co(NO₃)₂.6H₂O,(NH₄)₆Mo₇O₂₄.4H₂O, Fe(NO₃)₂.6H₂O or Ni(NO₃)₂.6H₂O as a catalyticallyactive metal precursor in distilled water and wet impregnating thesolution into a support, such as Al₂O₃, SiO₂ or MgO.

Specifically, the catalyst may be prepared by sonicating a catalyticallyactive metal precursor and a support, such as Al(OH)₃, Mg(NO₃)₂ orcolloidal silica.

Alternatively, the catalyst may be prepared by a sol-gel process. Inthis case, a chelating agent, such as citric acid or tartaric acid, isused to sufficiently dissolve a catalytically active metal precursor inwater. Alternatively, the catalyst may be prepared by co-precipitationof readily water-soluble catalytically active metal precursors.

The starting carbon nanotubes used in the method of the presentinvention may be prepared by bringing the supported catalyst intocontact with a carbon-containing compound in a heating zone.

The use of the supported catalyst prepared by an impregnation method ispreferred for the following reasons: the supported catalyst has a higherinherent bulk density than co-precipitated catalysts; unlikeco-precipitated catalysts, the supported catalyst produces a smallamount of a fine powder with a size of 10 microns or less, which reducesthe possibility of occurrence of a fine powder due to attrition duringfluidization; and high mechanical strength of the supported catalysteffectively stabilizes the operation of a reactor.

The catalyst may use at least one aluminum-based support selected fromthe group consisting of Al₂O₃, AlO(OH), Al(OH)₃, and mixtures thereof.The aluminum-based support is preferably alumina (Al₂O₃). The aluminum(Al)-based support may further include at least one oxide selected fromthe group consisting of ZrO₂, MgO, and SiO₂. The aluminum (Al)-basedsupport has a spherical or potato-like shape. The material for thealuminum (Al)-based support has a structure suitable to provide arelatively high surface area per unit weight or volume, such as a porousstructure, a molecular sieve structure or a honeycomb structure.

One embodiment of the present invention provides a method for preparinga supported catalyst for CNT synthesis, including (1) mixing a supportwith an aqueous metal solution including a catalytic component precursorand an active component precursor to prepare an aqueous solutioncontaining the supported catalyst precursors, (2) aging the aqueoussolution containing the supported catalyst precursors and impregnatingthe catalytic component precursor and the active component precursorinto the support to obtain a mixture, (3) drying the mixture undervacuum to coat the catalytic component and the active component on thesurface of the support, and (4) calcining the vacuum-coated product.

The use of the supported catalyst allows for the growth of carbonnanotubes by chemical vapor deposition through decomposition of a carbonsource, leading to the production of the carbon nanotubes.

Specifically, the chemical vapor deposition may be performed by feedingthe catalyst for carbon nanotube production into a fluidized bed reactorand introducing at least one carbon source selected from C₁-C₄ saturatedor unsaturated hydrocarbons, and optionally together with a mixed gashydrogen and nitrogen, into the reactor at 500 to 900° C. Carbonnanotubes are allowed to grow for 30 minutes to 8 hours after the carbonsource is introduced into the catalyst for carbon nanotube production.

The carbon source may be a C₁-C₄ saturated or unsaturated hydrocarbon.Examples of such hydrocarbons include, but are not limited to, ethylene(C₂H₄), acetylene (C₂H₂), methane (CH₄), and propane (C₃H₈). The mixedgas of hydrogen and nitrogen transports the carbon source, preventscarbon nanotubes from burning at high temperature, and assists in thedecomposition of the carbon source.

The use of the supported catalyst enables the preparation of carbonnanotubes in the form of a spherical or potato-like aggregate having aparticle size distribution (D_(cnt)) of 0.5 to 1.0. For example, in thecase where the catalyst is prepared by impregnating a catalyst componentand an active component into a spherical or potato-like granularsupport, followed by calcination, the catalyst has a spherical orpotato-like shape that is not substantially different from that of thesupport and a carbon nanotube aggregate grown on the catalyst also has aspherical or potato-like shape that is substantially the same as that ofthe support except for an increase in diameter. Herein, the spherical orpotato-like shape refers to a three-dimensional shape having an aspectratio of 1.2 or less, such as a sphere or ellipse.

The particle size distribution (D_(cnt)) of carbon nanotubes is definedby Expression 3:

D _(cnt) =[Dn ₉₀ −Dn ₁₀ ]/Dn ₅₀  [Expression 3]

where Dn₉₀, Dn₁₀, and Dn₅₀ are the number average particle diameters ofthe CNTs after standing in distilled water for 3 hours, as measuredunder 90%, 10%, and 50% in the absorption mode by using a particle sizeanalyzer (Microtrac), respectively.

The carbon nanotubes have a particle size distribution (D_(cnt)) of 0.55to 0.95, more preferably 0.55 to 0.9.

The carbon nanotubes may be of a bundle type or non-bundle type havingan aspect ratio of 0.9 to 1. Unless otherwise mentioned, the term“bundle type” used herein refers to a type of carbon nanotubes in whichthe carbon nanotubes are arranged in parallel or get entangled to formbundles or ropes, and the term “non-bundle or entangled type” describesa type of carbon nanotubes that does not have a specific shape such as abundle- or rope-like shape. The CNT bundles may have a diameter of 1 to50 μm.

The aspect ratio is defined by Expression 4:

Aspect ratio=the shortest diameter passing through the center of CNT/thelongest diameter passing through the center of CNT  [Expression 4]

The carbon nanotubes have a bulk density of 80 to 250 kg/m³.

Specifically, the bulk density is defined by Expression 5:

Bulk density=CNT weight (kg)/CNT volume (m³)  [Expression 5]

The present invention is characterized in that the density distributionof the carbon nanotubes is in a specific range.

The carbon nanotubes may have an average particle diameter of 100 to 800μm and a strand diameter of 10 to 50 nm.

The metal component remains in the form of a fine powder or impurity inthe carbon nanotubes with the above properties. The metal componentreacts with a chlorine compound in a high-temperature atmosphere to forma metal chloride having a lower boiling point than the metal component.The metal chloride is removed by evaporation at a temperature equal toor higher than the boiling point of the metal chloride. The purifiedcarbon nanotubes have improved physical properties, particularlyimproved thermal stability. Due to their improved physical properties,the purified carbon nanotubes are suitable for use in flame retardantmaterials and metal composites exposed to high temperature environments.

The present invention uses a method of removing residual metal generatedfrom a metal catalyst used in a manufacturing process of carbonnanotubes by chlorination of the residual metal by reacting with achlorine-containing compound at a high temperature. By purifying thecarbon nanotubes by using such a method, deterioration of physicalproperties due to metal impurities such as residual metals can beimproved.

The purification process of the carbon nanotubes will be described inmore detail.

The purification process of the carbon nanotubes comprises the steps of:

chlorinating a residual metal by reacting the metal remaining in theproduced carbon nanotubes with a chlorine-containing compound at a firsttemperature in a vacuum or inert gas atmosphere; and

evaporating and removing the chlorinated residual metal at a secondtemperature higher than the first temperature.

According to one embodiment, the chlorine-containing compound may bechlorine (Cl₂) or trichloromethane (CHCl₃) gas. Since thechlorine-containing compound is low in reactivity with the carbonnanotubes, the damage to the produced carbon nanotubes can be furtherreduced.

After the chlorination step, the evaporation and removal of thechlorinated metal at the second temperature may be carried out in aninert gas or a vacuum atmosphere for 30 minutes to 300 minutes. Thisshould be the range which only chlorinated residual metals can beremoved without affecting the carbon nanotubes. Further, the evaporationand removal of the chlorinated metal may proceed while alternatelyforming a vacuum atmosphere and an inert gas atmosphere, which mayfurther enhance the removal efficiency.

The content of metal impurity in the carbon nanotubes from which theresidual metal has been removed by the above method may be 50 ppm orless. The metal impurities in the carbon nanotubes may be measured byICP analysis. According to one embodiment, the carbon nanotube may be ametal catalyst containing a metal such as cobalt (Co) or iron (Fe) as amain component. In this case, after the purification, the content ofeach of main component metal may be 40 ppm or less and the total contentmay be 50 ppm or less.

The method of purifying carbon nanotubes as described above not only caneffectively remove residual metals such as catalytic metals whilesuppressing damage or cutting of carbon nanotubes or solidification ofcarbon nanotubes into amorphous carbon material, but also it cansuppress the occurrence of physical damage or cutting of carbonnanotubes due to purifying the carbon nanotubes without sonicating. As aresult, it is possible to provide carbon nanotubes having improvedmechanical properties and physical properties, and in particular, carbonnanotubes having remarkably improved thermal stability.

The present invention provides a method for producing a CNT compositematerial by using the carbon nanotube pellets.

The present invention can improve the problems of the change of thecontent generated by scattering of powders and safety issues by usingcarbon nanotubes in the form of pellet rather than carbon nanotubes inthe form of powder in composite materials. And since the density of thepellet form is higher than that of the powder form, transport, transferand improvement become easier. Therefore, it can be more effectivelyapplied to the manufacturing of composite materials.

For example, the composite material may be applied to an electrode of anelectrochemical storage device such as a secondary cell, a fuel cell ora super capacitor, an electromagnetic wave shield, a field emissiondisplay, or a gas sensor.

The present invention will be explained in more detail with reference tothe following examples, including comparative examples. However, theseexamples are provided for illustrative purposes only and are notintended to limit the scope of the invention.

Example 1

Distilled water was added to carbon nanotubes at a weight ratio of 5:1to 1:2, and was mixed for a predetermined time. Then, the mixture wasextruded by a screw extruder to prepare pellets.

Comparative Example 1

The carbon nanotubes used in Example 1 were used in the form of powders.

Comparative Example 2

The carbon nanotubes used in Example 1 were directly pressed at apredetermined pressure to prepare pellets.

Experimental Example 1: Comparison of Characteristics of Pellets

The particle sizes of the carbon nanotubes used in Example 1 andComparative Example 1 to Comparative Example 2 were measured anddescribed in Table 1 below.

In order to determine the dispersion characteristics according to themethod for producing carbon nanotube pellets, the carbon nanotubepellets of Example 1 and Comparative Example 2 were put into an NMPsolution and the degree and the time of dispersion were observed.

TABLE 1 Compression Shape of method sample D50 D90 D99 Example 1Extruder Pellet 140 293  437 Comparative none Powder 325 589  972Example 1 Comparative Press pellet 318 779 1827 Example 2

Table 1 and FIG. 2 show the particle sizes of the carbon nanotubes ofExample 1 and Comparative Examples 1 and 2. According to Table 1 andFIG. 2, for the carbon nanotubes contained in the pellets produced bythe dry method in Comparative Example 2, D50 is almost unchanged, butrather D90 increases. It is considered that because the pellets preparedby the dry method have poor dispersion characteristics in the solvent,the possibility of the carbon nanotubes aggregating with each otherincreases. Therefore, particles larger than the carbon nanotube powdermay exist in a range of the large particle size such as D90 as shown inTable 1 and FIG. 2. On the other hand, the particles of the carbonnanotubes dispersed from the pellets of Example 1 were reduced by 50% ormore as compared with the powdered carbon nanotubes. In FIG. 2, it canbe seen that the particle size is reduced overall.

FIG. 3 shows the results of dispersing the pellets prepared in Example 1and Comparative Example 2 in a solvent. As shown in FIG. 3, it was foundthat the pellets of Example 1 were dispersed in the solvent at the sametime as introduction of the solvent, but the pellets of ComparativeExample 2 produced by dry compression were not dispersed immediatelyupon introduction of the solvent. It can be seen from the graph ofparticle size distribution of FIG. 2 that there are some large particleseven after dispersion.

Example 2

Carbon nanotubes and distilled water were mixed for 2 hours to prepare afirst paste (Kneading). Then, distilled water was added to the firstpaste and mixed them for 1.5 hours to prepare a second paste (Paste).Distilled water was further added to the second paste and the mixturewas diluted to some extent (Dilution). Thus, premixing process of thecarbon nanotubes was carried out. The viscosity of the premixed carbonnanotube paste was 7,000 cps. Table 2 shows the particle size of thecarbon nanotubes according to the steps of the premixing process and themixing time in each step. FIG. 4 shows a change of particle sizedistribution according to time of the premixing process (FIG. 4a ) and achange of particle size of D50 and D90 according to the progress ofprocess (FIG. 4b ).

The premixed carbon nanotube paste was introduced into a screw mixer.The discharging rate of the screw mixer was 0.5 kg/min, and theviscosity of the mixed solution of the carbon nanotube discharged was22,000 cps.

TABLE 2 Content of Mixing solids RPM D50/D90 Dispersion Step (%) MixingTime (H/L) (μm) Effect First 8.89 30 min 1500/160 39.6/89.1 — (Kneading)8.44 +30 min(1.0 h) 15.1/46.1 D50/D90 reduction +30 min(1.5 h) 14.1/45.7— 8.04 +30 min(2.0 h) 13.7/47.3 — Second 5.62 +30 min(2.5 h) 1100/140 7.3/38.7 — (Paste) +30 min(3.0 h)  7.0/34.2 D50/D90 reduction +30min(3.5 h)  7.3/38.6 — Third 2.40 +30 min(4.0 h) 500/60  7.0/37.2(Dilution)

Example 3

Carbon nanotube pellets prepared by extruding the paste produced inExample 2 and distilled water were mixed for 2 hours to prepare a firstpaste (Kneading). Then, distilled water was added to the first paste andmixed them for 1.5 hours to prepare a second paste (Paste). Distilledwater was further added to the second paste and the mixture was dilutedto some extent (Dilution). Thus, premixing process of the carbonnanotubes was carried out. The viscosity of the premixed carbon nanotubepaste was 20,000 cps. Table 3 shows the particle size of the carbonnanotubes according to the steps of the premixing process and the mixingtime in each step. FIG. 5 shows a change of particle size distributionaccording to the premixing process and the premixing time (FIG. 5a ) anda change of particle size of D50 and D90 according to the progress ofprocess (FIG. 5b ).

The premixed carbon nanotube paste was introduced into a screw mixer.The discharging rate of the screw mixer was 0.5 kg/min, and theviscosity of the mixed solution of the carbon nanotube discharged was9,000 cps.

TABLE 3 Content of Mixing solids RPM D50/D90 Dispersion Step (%) MixingTime (H/L) ^(a)) (μm) Effect First 8.89 30 min 1500/160 18.1/43.4 —(Kneading) +30 min(1.0 h) 13.4/36.2 D50/D90 reduction +30 min(1.5 h)11.7/34.0 D50/D90 reduction +30 min(2.0 h) 10.4/32.1 D50/D90 reductionSecond 5.62 +30 min(2.5 h) 1100/140 5.4/23  — (Paste) +30 min(3.0 h)5.4/23  D50/D90 reduction +30 min(3.5 h) 5.5/24  — Third 2.40 +30min(4.0 h) 500/60  5.6/23.7 (Dilution) ^(a)) RPM H/L: rpm of thevertical direction/rpm of the horizontal direction

Experimental Example 2: Changes in Particle Size and Viscosity DuringPelletizing of CNT Powders and of Pressed CNTs

Table 4 and FIG. 6 show changes in particle size in the premixing ofExample 2 and Example 3 in order to compare the particle size change andthe viscosity characteristics with use of CNT powders and compressedCNTs in the production of carbon nanotube pellet. Table 7 shows aparticle size distribution (FIG. 7a ) of the premixed carbon nanotubeand the SEM image (FIG. 7b ) of the shape of the carbon nanotubes afterdischarging from the screw mixer.

TABLE 4 Particle size (D50/D90, μm) Viscosity (cps) @ 12 rpm FirstSecond Third ^(b))S/M S/M (Kneading) (Paste) (Dilution) 1Pass Pre- 1pass (2.0 hr) (5 hr) (0.5 hr) (0.5 Kg/min) mixing (0.5 Kg/min) Example 213.7/47.3 7.3/38.6 7.0/37.2 7.17/26   7000 22000 Example 3 10.4/32.15.5/24  5.6/23.7 5.9/20.5 2000 9000 ^(b))S/M 1Pass: First dischargingprocess from the screw mixer

According to Table 4 and FIG. 6, when compressed CNTs, i.e., pelletizedcarbon nanotubes are used, the particle size of the CNTs in thepremixing step is smaller than that of uncompressed CNTs, which meansthat the particle size of the CNTs is reduced by the pelletizingprocess. However, in the kneading step, which is an initial firstkneading process, the CNT particles which are discharged from the screwmixer are observed to be larger than the uncompressed CNT particles,which may be due to agglomeration of the compressed CNT particles in theextrusion process. It is appeared that in the second and third steps theparticle size gradually decreases and remains almost constant. This isconsidered to be due to disassembling of the CNT particles agglomeratedby the compression process. In addition, the viscosity in the premixingand the viscosity after the S/M discharging for Example 3 usingcompressed CNTs were significantly decreased as compared with Example 2.These properties may allow to improve the dispersion property of CNT inthe process. This may be an effect of increasing the density of each ofthe CNTs as the particle size of the CNTs having the same mass isreduced by the compression process.

FIG. 9 shows shapes of distributions of (a) CNTs and (b) pressed CNTsafter premixing process.

Experimental Example 3: Increase of Discharging Rate

Carbon nanotube pellets prepared by extruding the paste produced inExample 2 and distilled water were mixed for 2 hours to prepare a firstpaste (Kneading). Then, distilled water was added to the first paste andmixed them for 1.5 hours to prepare a second paste (Paste). Distilledwater was further added to the second paste and the mixture was dilutedto some extent (Dilution). Thus, premixing process of the carbonnanotubes was carried out. The particle size and the viscosity of thepremixed carbon nanotube paste are shown in Table 5.

The premixed carbon nanotubes were introduced into a screw mixer anddischarged at a discharge rate of 2.0 kg/min. The viscosity and theparticle size of the carbon nanotube paste which is first discharged areshown in Table 5.

The first discharged carbon nanotube paste was reintroduced into thescrew mixer and discharged at a discharge rate of 2.0 kg/min. Theviscosity and the particle size of the carbon nanotube paste which issecond discharged are shown in Table 5.

FIG. 8 shows a graph of the particle size distribution of each step(FIG. 8a ) and the SEM image of the shape of distribution of the carbonnanotubes (FIG. 8b ).

TABLE 5 Particle size (μm) Viscosity (cps) D50 D90 @12 rpm Premixing5.61 23.7 2000 S/M 1Pass 6.34 27.8 5250 S/M 2Pass 6.19 22.5 6333

As shown in Table 5, the change of the particle size according to therepetition of the discharging process does not appear much, and thetendency of the particle size of D90 to decrease slightly may appear dueto the effect of disassembling of agglomerates by the stirring andmixing. In addition, as the discharging rate increases, the dispersioncharacteristics of the carbon nanotubes may deteriorate, which may beattributable to an increase in viscosity of paste.

Experimental Example 4: Comparison of the Change of the Particle Sizewith or without Premixing Process Example 4

Carbon nanotube powders and distilled water were mixed for apredetermined time. The mixture was introduced into a screw mixer andmixed for a predetermined time. Then, the mixture was first extrudedwith an extruder to prepare pellets.

The first extruded pellets were mixed with distilled water again. Themixture was introduced into the screw mixer and mixed for apredetermined time. Then, the mixture was second extruded with theextruder to prepare pellets.

The second extruded pellets were mixed with distilled water again. Themixture was introduced into the screw mixer and mixed for apredetermined time. Then, the mixture was third extruded with theextruder to prepare pellets.

The particle sizes of the carbon nanotubes contained in the pellets ineach of the first, second and third extrusion steps were measured andthe results are shown in Table 6 below. The particle size in each stepwas measured by dispersing each pellet in an NMP solution.

FIG. 10 shows a comparison of the particle size distributions accordingto the number of times of passing through the extruder.

Example 5

Carbon nanotubes and distilled water were mixed for 2 hours to prepare afirst paste (Kneading). Then, distilled water was added to the firstpaste and mixed them for 1.5 hours to prepare a second paste (Paste).Distilled water was further added to the second paste and the mixturewas diluted to some extent (Dilution). Thus, premixing process of thecarbon nanotubes was carried out.

The premixed solution of carbon nanotubes was introduced into a screwmixer, mixed for 30 minutes, and then extruded with an extruder toprepare pellets.

The first extruded pellets were mixed with distilled water again. Themixture was introduced into the screw mixer and mixed for apredetermined time. Then, the mixture was second extruded with theextruder to prepare pellets.

The second extruded pellets were mixed with distilled water again. Themixture was introduced into the screw mixer and mixed for apredetermined time. Then, the mixture was third extruded with theextruder to prepare pellets.

The particle sizes of the carbon nanotubes contained in the pellets ineach of the first, second and third extrusion steps were measured andthe results are shown in Table 6 below. The particle size in each stepwas measured by dispersing each pellet in an NMP solution.

FIG. 11 shows the particle size distribution according to the number oftimes of passing through the extruder of the premixed CNTs.

TABLE 6 Example 4 Example 5 D50 (μm) D90 (μm) D50 (μm) D90 (μm) Powder419 664 7.3 36.7 1^(st) passing 184 528 5.6 22.6 2^(nd) passing 120 265— — 3^(rd) passing  90 194 5.9 26.6

As shown in Table 6, in the case of the pelletized carbon nanotubeswithout the premixing, the initial particle size of the carbon nanotubesgradually decreases in proportion to the number of times of passingthrough the extruder. However, in the case of the carbon nanotubes whichhave passed through the extruder after premixing, the change of theparticle size according to the passage of the extruder does not appearmuch. This is because in the premixing process the agglomerates of thecarbon nanotubes which were physically aggregated are sufficientlydisassembled and the carbon nanotubes in the disassembled state arepelletized, so that the change of the particle size according to thepassage of the extruder can be small or almost not. On the other hand,in the pelletizing process without the premixing, the agglomerates ofthe carbon nanotubes are compressed, so that the size of theagglomerates may be decreased according to the number of times ofpassing through the extruder. Therefore, the particle size tends todecrease according to the number of times of passing through theextruder. For example, D90 of the particle size of the carbon nanotubesmay satisfy the following Expression 2:

−167.3x+650≤y≤−167.3x+670  [Expression 2]

wherein x is the number of times of extrusion of the carbon nanotubes,and y is the particle size of D90 (μm) of the carbon nanotubes.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that suchdetailed descriptions are merely preferred embodiments and the scope ofthe present invention is not limited thereto. Therefore, the true scopeof the present invention should be defined by the appended claims andtheir equivalents.

1. An apparatus for manufacturing carbon nanotube pellets comprising: amixing part having a mixing device for mixing carbon nanotubes and asolvent; a kneading part provided at a bottom of the mixing part and foradditionally kneading a mixture of the carbon nanotubes and the solventto prepare a carbon nanotube paste; and an extruding part for receivingthe mixture from the kneading part and molding the mixture into pelletsby compression molding.
 2. The apparatus for manufacturing carbonnanotube pellets according to claim 1, wherein the kneading part isintegrated into the mixing part.
 3. The apparatus for manufacturingcarbon nanotube pellets according to claim 1, wherein the mixing partand the kneading part are isolated from each other, the carbon nanotubesare mixed in the mixing part and sequentially the mixture is supplied tothe kneading part, and the mixing by the mixing part and the kneading bythe kneading part are independently performed.
 4. The apparatus formanufacturing carbon nanotube pellets according to claim 3, wherein ahopper is provided at the bottom of the mixing part, the mixing part andthe kneading part are isolated from each other by the hopper, the carbonnanotube mixture mixed in the mixing part is supplied to the kneadingpart via the hopper and the mixing by the mixing part and the kneadingby the kneading part are independently performed.
 5. The apparatus formanufacturing carbon nanotube pellets according to claim 1, wherein thekneading part is a screw mixer having at least one axis.
 6. Theapparatus for manufacturing carbon nanotube pellets according to claim1, wherein the extruding part further comprises a molding part formolding extrudate into the pellets having a predetermined length anddiameter.
 7. A method for producing carbon nanotube pellets by using theapparatus of claim
 1. 8. The method for producing carbon nanotubepellets according to claim 7, wherein the method comprises the steps of:mixing carbon nanotubes and a solvent at a weight ratio of 5:1 to 1:2 inthe mixing part; kneading the mixture to prepare a carbon nanotube pastein the kneading part; extruding the carbon nanotube paste to mold intopellets in the extruding part; and collecting and drying the moldedpellets.
 9. The method for producing carbon nanotube pellets accordingto claim 8, the method further comprises the step of adding a solvent tothe mixture of carbon nanotubes and solvent.
 10. Carbon nanotube pelletsproduced by using the apparatus of claim 1.